In a production of acetic acid, when a solution containing water, hydrogen iodide, methyl iodide, methyl acetate, acetic acid, and others is distilled for purification, hydrogen iodide forms an azeotrope with water. As a result, hydrogen iodide is condensed in a distillation column, thereby it is difficult to efficiently remove hydrogen iodide even when the mixture is distilled. Moreover, in the case where the concentration of hydrogen iodide in a distillation column is high, corrosion of the column body is accelerated.
Therefore, in order to reduce the concentration of hydrogen iodide in the distillation column, it has been proposed to convert hydrogen iodide into methyl iodide by feeding (or introducing) a component such as methanol to a middle plate of the distillation column.
Japanese Patent Application Laid-Open No. 40999/1994 (JP-6-40999A) (Patent Document 1) discloses a process for producing acetic acid, which comprises supplying methanol and carbon monoxide to a carbonylation zone that holds a liquid reaction composition consisting of a rhodium catalyst, methyl iodide, an iodide salt, water having a content up to about 10% by weight, methyl acetate having a concentration of at least 2% by weight, and acetic acid; introducing the liquid reaction composition into a flash zone; and recycling a liquid component from the flash zone to the reaction zone and collecting an acetic acid product from a vapor fraction of the flash zone by using simple distillation; wherein the vapor fraction from the flash zone is introduced into the distillation zone, a light end stream to be recycled is removed from the top of the distillation zone, and an acid stream which has a water content of less than 1500 ppm and a propionic acid concentration of less than 500 ppm is withdrawn from the distillation zone. The document also mentions that an iodide as an impurity is removed from the acetic acid product by passing the acetic acid product through an ion-exchange resin (anion-exchange resin) bed. Further, this document describes that increase of the hydrogen iodide component is inhibited by introducing a small amount of methanol into the distillation zone (preferably the lower part of the supplying point thereof), converting hydrogen iodide into methyl iodide, and removing the converted matter into the light end stream to be recycled; that the process can treat hydrogen iodide up to 5000 ppm in the feeding matter; and that a relatively high concentration of methyl acetate in the distillation zone converts hydrogen iodide into methyl iodide by applying the distillation zone at a sufficiently high pressure to remove methyl iodide into the light end stream to be recycled.
However, even in these processes, since removal of hydrogen iodide depends on a reaction with methanol or methyl acetate, hydrogen iodide cannot be efficiently removed. Moreover, it is not preferred to increase the pressure for converting hydrogen iodide into methyl iodide, because corrosion due to hydrogen iodide is promoted. Incidentally, even in the case of removing the acetic acid product containing water of not more than 1500 ppm, the concentration of hydrogen iodide in the acetic acid product cannot be greatly reduced because of affinity between hydrogen iodide and water. Furthermore, in order to further reduce the concentration of hydrogen iodide, an anion-exchange resin is required, and as a result, processing cost increases.
Japanese Patent Application Laid-Open No. 23016/1977 (JP-52-23016A) (Patent Document 2) discloses a process for removing and collecting an iodine-containing component and drying acetic acid, which comprises introducing an acetic acid stream containing water, methyl iodide, and hydrogen iodide into a middle point of a first distillation zone, removing a major portion of methyl iodide and a part of water from the top of the first distillation zone, removing a major portion of hydrogen iodide from the bottom of the first distillation zone, withdrawing a stream from a center part of the first distillation zone to introduce the withdrawn stream into an upper part of a second distillation zone, introducing a methanol stream into a lower part of the second distillation zone, removing a stream containing the residual water and methyl iodide which are coexistent with methyl acetate from the top of the second distillation zone, and withdrawing a stream comprising an acetic acid product, which is substantially dried and substantially free from hydrogen iodide and methyl iodide, from the bottom or near the bottom of the second distillation zone. This document also discloses that, in a process for recovering acetic acid by distilling a side-cut fraction from the first distillation column in the second distillation column, it is unnecessary to recycle a fraction containing hydrogen iodide as the side-cut fraction of the second distillation column to the first distillation column by introducing methanol into the second distillation column and chemically removing hydrogen iodide.
However, in order to reduce the concentration of hydrogen iodide, methanol introduced into the second distillation column is necessary to be reacted with hydrogen iodide. Accordingly, it is difficult to efficiently reduce the concentration of hydrogen iodide with simple distillation operation.
Great Britain Patent No. 1350726 specification (Patent Document 3) discloses a purification process of a monocarboxylic acid component containing water and alkyl halide and/or hydrogen halide contaminants, which the process comprises introducing a monocarboxylic acid component containing water and alkyl halide and/or hydrogen halide contaminants into the upper half of a distillation zone, removing an overhead fraction containing a major proportion of water and the alkyl halide, removing a stream from the middle portion of the zone and below the point of the introduction to eliminate a major proportion of hydrogen halide present in the zone, and removing a product monocarboxylic acid stream from the bottom part of the zone, the product acid stream being substantially dry and substantially free of any alkyl halide and hydrogen halide contaminants. The document describes that a concentration of hydrogen halide peaks in the middle portion of the distillation column where the liquid composition of carboxylic acid has a water content from about 3 to 8 percents, and if a side stream is withdrawn from the middle portion of the distillation column, a monocarboxylic acid in which almost all of hydrogen halide is removed can be obtained. Further, the document also discloses that in the case where a reaction product obtained from a reaction of methanol with carbon monoxide is subjected to a flash distillation, and a fraction separated by the flash distillation is introduced into the distillation column, hydrogen iodide is condensed in a side stream from the middle portion of the distillation column and removed.
However, in such a process, not only irregular distillation off of hydrogen iodide from a bottom solution or side stream cannot be avoidable because of fluctuation of the peak position of hydrogen halide concentration due to variable factors (such as temperature, and pressure) in the distillation step, but also the quality of acetic acid sometimes fluctuates because of contamination with hydrogen iodide of an acetic acid product. Further, due to affinity between hydrogen iodide and water, there is a limit to reduce the hydrogen iodide concentration in the acetic acid product.
Patent Document 1: JP-6-40999A (Claims, and Paragraph No. [0043])
Patent Document 2: JP-52-23016A (Claims, the lower right column of page 5, the lower light column to the lower right column of page 7, and Example 1)
Patent Document 3: Great Britain Patent No. 1350726 specification (Claims, page 2, lines 66-76, and Example 1)